1. Field of the Invention
The present invention relates to methods and apparatus (a system) for shaping and polishing (finishing) a surface, more particularly to methods and apparatus for shaping and polishing a surface by the impingement of an abrasive jet, and most particularly to methods and apparatus for shaping and polishing a surface by the impingement of a magnetically-modifiable and magnetically-directable jet.
2. Discussion of the Related Art
Water jets containing abrasive particles are known to be used for cutting or shaping materials such as glass, ceramics, plastics and metals. This technology is known generally as abrasive stream finishing, or abrasive suspension jet machining, or abrasive flow machining. Typically, such jets are impinged upon the substrate to be cut at a relatively high velocity, which may exceed 10 meters per second. When the jet strikes the impact zone, the abrasive particles in the water carrier chip away particles of the substrate surface. The rate of material removal is a function of the kinetic energy of the jet, the sharpness, size, and hardness of the abrasive particles the material of the substrate, the distance from the jet nozzle to the workpiece, and the angle of incidence of the jet.
It has been difficult in practice to adapt abrasive liquid jet technology for precision finishing of surfaces of highly-demanding objects such as, for example, optical components. A fundamental property of a fluid jet is that it begins to lose its collimation as the jet exits a nozzle, due to a combination of abruptly imposed longitudinal and lateral pressure gradients, surface tension forces, and aerodynamic disturbance. A water jet tends immediately to spread out and to break into droplets within a short distance of a nozzle, typically within a few nozzle diameters of the nozzle orifice, increasing thereby the cross-sectional area of the jet and proportionally decreasing the unit kinetic energy within the jet. For this reason, the nozzle of an abrasive cutting jet typically is situated as close as is practical to the workpiece to be cut. Reducing pressures and flow rates to place the jet in a flow regime where it can polish rather than cut also serves to degrade the jet further so that it is not readily concentratable on a small area of the workpiece. Increasing the viscosity of the cutting medium by addition of viscosity-building agents can help to stabilize the jet but also proportionally increases the resistance to fluid flow in the delivery system and the pumping power required to deliver the fluid to the nozzle, making impractical a high speed, high-viscosity jet for either cutting or polishing.
A further limitation of using an abrasive water jet for polishing is that the jet is positionable against the workpiece only by adjusting either the attitude of the nozzle or the position of the workpiece. The jet itself cannot be redirected or guided once it leaves the nozzle orifice.